Vaporized material source



Filed Aug. 4,

A. R. HAMILTON VAPORI ZED MATERIAL SOURCE 2 Sheets-Sheet l AWM M, w67 A. R. HAMILTON SISAM VAPORIZED MATERIAL SOURCE Filed Aug. 4, 1964 2 Sheets-Sheet 2 United States Patent O 3,313,474 VAEREZEB MATERIAL SOURCE iien R. Hamiiton, Rochester, N.Y., assigner to Consc-iidated Vacuum Corporation, Rochester, NX., a corporation otr New York Filed Aug. 4, 1964, Ser. No. 387,410 7 Claims. (Cl. 230--69) The present invention relates to apparatus and equipment for vaporizing material and, more particularly, to a source of vaporized material.

Sources of vaporized material are well known. They are, for example, used in vacuum pumps for providing getter material inside the pump housing, and in vacuum coaters for depositing various materials on articles. Thus, the expression vaporized material as used herein is meant to cover at least vaporized getter material, as well as vaporized materials of the type used in vacuum coating processes.

An advanced type of vaporized material source comprises a rod made of material to be vaporized and mounted to have a free end, and electron releasing means, such as a filament, located in the vicinity of the free end of the rod. The source includes means for varying the spacing between the free end of the rod and the electron releasing means, and also an electric circuit including means for positively biasing the rod with respect to the electron releasing means for causing electron bombardment of the free end of the rod and vaporization of material therefrom.

In the operation of this type of source, the position of the rod is initially adjusted to realize a predetermined initial spacing between the free end of the rod and the electron releasing means, as dictated by the current and voltage capability of the bombardment power supply and the electron capability of the filament. As vaporization continues, the rod is progressively worn down and the spacing between the free rod end and the electron releasing means has to be adjusted and restored from time to time. Between these adjustments, there are periods of time during which the rate of material vaporization is different from the desired rate.

The present invention overcomes this disadvantage by providing means for effecting these adjustments automatically. According to one aspect of the invention, these means comprise means for sensing the current in the above-mentioned electric circuit and means responsive to variations in this current for adjusting the spacing between the free end of the rod and the electron releasing means.

A preferred embodiment of the invention includes means for producing a voltage being proportional to the current in the above-mentioned electric circuit, means for producing a reference voltage, means for producing an error signal by comparing the current proportional voltage and the reference voltage, and means responsive to the error signal for adjusting the position of the rod with respect to the electron releasing means. Since the abovementioned current varies with the latter spacing and the error signal with this current, the spacing between the free rod end and the electron releasing means is automatically adjusted. It is within the scope of the invention that the above-mentioned reference voltage may be adjustable. In this case, the spacing -between the free rod end and the electron releasing means can be set to a desired value during the operation of the source simply by varying the reference voltage.

Another embodiment of the invention includes also means for producing a voltage being proportional to the current in the above-mentioned circuit and means for producing a reference voltage. In addition, this latter embodiment includes means for producing an error signal when the reference voltage exceeds the current responsive ice voltage, and means for moving the rod toward the electron releasing means by a predetermined increment of linear motion in response to the error signal. In this case, the spacing between the free rod end and the electron releasing means is automatically adjusted by a predetermined amount, whenever the current in the above-mentioned circuit indicates that this spacing has grown too large due to wear of the rod of vaporizable material.

The invention will become more readily apparent from the following detailed description of preferred embodiments thereof, illustrated by Way of example in the accompanying drawings, in which:

FIG. 1 is an elevation, partially in section, of a getter-ion vacuum pump embodying the invention;

FIG. 2 is a diagram of a control circuit embodying a modification ofthe invention; and

FIG. 3 is a diagram of .a control circiut embodying a further modification of the invention.

The getter-ion pump 10 shown in FIG. 1 comprises a pump housing 11 having a substantially cylindrical side wall 12, an upper flange 13 encompassing an inlet opening 14, and a bottom 15 defining a central aperture 16. An .anode grid 18 is located in pump housing 11 as shown.

The anode grid 18 is positively biased with respect to pump housing 11 by an ionizing high voltage supply 20. The voltage supply 20, which includes transformer and rectifier means and may be 0f a conventional type, is energized from a pair of power leads 22 and 23 which are connected through an on-off switch .24 to a conventional source (not shown) of .alternating current power. A lead 26 extends through an insulating bushing 27 in housing wall 12 and connects the positive output terminal of source 20 to the anode grid 18. The negative terminal of source 20 is connected to housing wall 12 lby a lead 28. A conventional electron-emitting filament 19 is associated with anode 18 and is energized from power leads 22 and 23 through a filament transformer 21.

In the `operation of the pump, the inlet 14 is connected to a space or vessel (not shown) to be evacuated. The voltage provided by supply 20 causes attraction of electrons from filament 19 to anode grid 18. As is well known, these electrons are effective to ionize gas molecules present in pump housing 10. These ionized gas molecules deposit themselves on the inner wall surface 30 which thus acts as a gas molecule collector. In this manner, gas is pumped from the space or vessel to be evacuated.

To enhance this pumping action, the vacuum pump includes a source 32 of getter material which comprises a rod 33 of getter material, such as titanium, and an electron releasing filament 34 located above the free end 35 of the getter rod. A heating current lead 37, which extends through .an insulating bushing 38 in housing wall 12, connects one end of filament 34 to one end of the secondary winding 46 of a filament transformer 41. A second heating current lead 42, which extends through an insulating bushing 43 in housing wall 12, connects the other end of iilament 34 to the other end of secondary winding 40. The primary winding 45 of transformer 41 is connected to the alternating current power leads 22 and 23.

The getter rod is connected on a support 47 and is positively `biased with respect to the filament 34 by a high voltage, direct-current supply 48 which includes transformer and rectifier means and may 'be of a conventional type. Voltage supply 48 is energized from the alternating current power leads 22 and 23. The negative output terminal 49 of voltage supply 48 is connected by a lead 50 to the center tap 51 of the filament transformer secondary winding 40. An electric circuit 53, including a resistor 55, .a milliammeter 56 with termnials 57 and 58, a rigid lead 59 extending through an insulating bushing 60 in housing bottom 15, and a fiexible lead 61 attached to support 47, connects the getter rod 33 to the positive terminal 62 of the voltage supply 4S.

In the operation of the getter supply 32, the position of the rod 33 is first adjusted in the manner described below, so that the free rod end 35 has a predetermined initial spacing with respect to filament 34. The filament transformer 41 and the voltage supply 48 are then energized to cause the free rod end 35 to be bombarded with electrons of suicient number and energy to bring about vaporization of getter material from the free rod end 35. In the apparatus shown in FIG. 1, a hollow cylindrical shield 63 is located to encompass the `getter rod 33. A lead 64, which extends through an insulating lbushing 65 in housing wall 12, connects the shield 63 to the abovementioned lead and thus to the negative terminal 49 of voltage supply 48. The shield 63 thus aids in directing and focussing electrons onto the free rod end 35.

The rod support 47 is connected on a post 65 which is preferably of electrically and thermally insulating material. The post 65 is connecte-d on the bottom plate 66 of a bellows 67 which is sealably attached by means of a fiange 68 to the housing =bottom 15. A spindle receptacle 70 is attached to the lower surface of bottom 66 and eX- tends through an opening 71 in a yoke 72 which is welded to housing bottom 15. The spindle receptacle 70 has a longitudinal key 73 which sl-ides in -a corresponding key Way (not shown) in yoke 72 to prevent rotary movement of spindle receptacle 70. A reduction gear 75 is mounted on yoke 72 by a number of posts 76. The gear has an input shaft 77 and an output shaft 78 which comprises a spindle 79 threaded into spindle receptatcle 70. A servo motor 80 is mounted on gear 75 by a number of posts 81. The motor 80 has a shaft 83, one end of which is connected to gear input shaft 77 by a coupling 84. The other end of motor shaft 73 is provided with a knurled knob 85. Rotation of knob will cause rotation of motor shaft 83 and thus of `gear outputshaft 78. The spindle 79 and the spindle Vreceptacle 70 will translate this rotary motion into a linear motion producing a positioning force on bottom 66 of bellows 67. In this manner, the spacing between the free end 35 of getter rod 33 and the filament 34 may be set to a desired initial value by rotating knob 85, either manually or by means of an electric auxiliary motor (not shown). The milliammeter 56, which measures the current in circuit 53, will indicate when the desired spacing has been realized, since this current varies with the latter spacing.-

During the operation of the getter supply 32, the current which fiows through circuit 53 produces across resistor 55 a voltage V corresponding to this current. This voltage is compared with a reference voltage VE occurring between the movable arm and one end 91 of a potentiometer 92. To this effect, one end of resistor 55 is connected to movable potentiometer arm 90 by a lead 94, and the end 91 of potentiometer 92 is connected to a term-inal 95 by a lead 96. A lead 97 with switch 105 connects the other end of resistor 55 to a terminal 98. The potentiometer 92 is connected across the output terminals of a reference voltage supply in such a manner that the reference voltage VE acts in opposition to the voltage V across resistor 55. In this manner, an error signal or voltage VE will appear across terminals 95 yand 98 and will `be equal to the difference between voltage V and voltage VE. The reference voltage supply is energized from the alternating current and power leads 22 and 23, and may be of la conventional type including the usual transformer, rectifier and voltage stabilizer means.

The error voltage VE across terminals 95 and 98 is applied to the input leads 191 and 102 of servo motor 80. The motor 80 is of a type developing one sense of rotation .at its shaft 83 when one of the leads 101 and 102 is positive with respect to the other lead, and developing the other sense of rotation at its shaft 83 when the other one of its leads 101 and 102 -is positive with respect to the aforesaid one lead. Servo motors of this type are well known.

The movable arm 90 of potentiometer 92 is initially adjusted so that the reference voltage VR is equal in magnitude to the voltage V which occurs across resistor 55 when the spacing between rod end 35 and filament 34 is as desired. As long as this desired spacing persists, the error voltage VE between terminals 95 `and 98 will be zero and the servo motor S0 will not `be energized. As the getter -rod 33 is worn down, the spacing between rod end 35 and filament 34 will increase and the voltage across resistor 55 will decrease due to `decreasing current in circuit 53. Therefore, there will be an error voltage between terminals 95 and 98, with the terminal 98 being positive with respect to the terminal 95. In the illustrated embodiment it is assumed that an error voltage of this polarity will cause the motor 80, gear 75 Iand spindle 79 to move the rod 33 toward filament 34. Should this not be the case, it will merely be necessary to connect motor lead 102 to terminal 95 and motor lead 101 to terminal 98. As the free rod end 35 approaches filament 34, the current through resistor 55 will increase and the error voltage VE will Iagain approach zero magnitude. Should the current through resistor 55 `become too high, the error voltage VE will be reversed in polarity and the motor 80 will turn in the opposite direction causing the free rod end 35 to move away from filament 34.

In this manner, the desired spacing between getter rod 33 and filament 34 is maintained automatically while getter material is vaporized from the rod 33. If it is desired to change this spacing, the movable potentiometer arm 90 is adjusted until a new equilibrium between voltage V and reference voltage VR is reached. The switch 105 in lead 97 is opened when the operation of motor 80 is not desired, such as during the initial adjustment of the spacing between `getter rod end 35 and filament 34. A switch (not shown) may also be provided in power leads 22 and 23 so as to permit de-energization of transformer 41, and supplies 48 yand 100, when the operation of the -getter supply 32 is not desired.

The vaporized getter material enters the space in pump housing 11 and deposits itself on inner wall surface 30 where it aids in the entrapment of gas molecules. If desired, the yanode grid 18 may be located farther away from getter source 32 than -as shown in FIG. l, so that the getter material will deposit itself on portions of the inner housing wall that are located below anode grid 13.

With reference to FIG. l, it will of course be appreciated that this figure represents the apparatus shown therein in a Vsomewhat simplified fashion, and that many modifications of this apparatus are possible within the scope of the invention.

FIG. 2 shows an electric control circuit which may be employed with the apparatus shown in FIG. 1 or with a modification thereof. This control circuit has a pair of terminals 111 and 112 which are connected to the terminals 95 and 98 shown in FIG. 1, in lieu of the motor leads 101 and 102. In other words, the servo motor 80 shown in FIG. 1 is disconnected from the terminals 95 and 98 and the terminals 111 and 112 are connected to terminals 95 and 98, so that the error signal or voltage VE appears across terminals 111 and 112 shown in FIG. 2. The servo motor 80 shown in FIG. 1 is replaced by an alternating current motor 114 shown in FIG. 2, the shaft of which is connected to the input shaft 77 of the gear 75 shown in FIG. l. A relay 115 is connected to terminals 111 and 112 through a unidirectional current conducting device or rectifier cell 116. The cell 116 is poled s0 that the relay 115 will only be energized when the voltage V across the resistor 55 shown in FIG. 1 falls below the reference voltage VE. The relay has a Contact 117 which, when closed, completes a circuit extending from a power lead 118 through a timer motor 119, contact 117 and to a power lead 120. The power leads 118 and 120 are connected to a conventional sourcel of alternating current power. The timer motor 119 is of a conventional type and has a shaft 122 which is connected to a reduction gear 123. The gear 123 drives two cam discs 125 and 126 in the direction of the arrows shown on these discs. The cam disc 126 has a normally open contact 123 associated therewith. The cam disc 126 also has a projection 1313 of a predetermined circumferential length for closing the contact 12S for a predetermined time during every revolution of cam disc 126. The contact 123, when closed, completes a circuit from power lead 118, through motor 114, contact 128 and to power lead 129. In this manner, the motor 114 is energized for a predetermined time during every revolution of cam disc 126. The cam disc 125 serves to assure that the cam disc 126 completes at least one full revolution every time the relay 115 is energized. To this etect, the cam disc 125 has an indentation 132 in which there rests an actuating finger 134- of a normally closed contact 135 which is connected in parallel to relay contact 117. When the cam disc 125 is caused to move from its illustrated position, it pushes the actuator linger 134 out of its indentation 132 and thus closes contact 135. The energization of timer motor 119 will then continue irrespective of whether the relay contact 117 opens or not before the cam disc 125 has completed a full revolution. When this cam disc 125 has completed a full revolution, the actuator linger 134 will drop back into the indentation of cam disc 125 and the contact 135 will open. This will cause de-energization of timer motor 119, provided the relay Contact 117 has opened in the meantime. if the relay contact 117 is still closed, the discs 125 and 126 will commence another revolution and will complete the last full revolution in a given cycle after the relay Contact 117 has opened.

The operation of the control circuit shown in FIG. 2 can be summarized as follows:

As long as the voltage V across resistor 55 of FiG. 1 is larger than the reference voltage VR indicated in FG. 1, the rectifier cell 116 prevents the operation of the relay 115 0f the circuit shown in FIG. 2. The relay 115 will also not be energized if the voltage V is equal to the reference voltage VR, since the error voltage VE is then zero. If the spacing between the end of getter of 33 and the lament 35 increases due to wear of the getter rod, the current in circuit 53 of FiG. l decreases. This leads to a decrease in the voltage V across resistor 55 and, in consequence, to an error voltage VE which is poled so as to be able to energize relay 115 through the rectier cell 116. The relay 115 will thus close its contact 117, which will cause energization of the timer motor 119. The cam discs 125 and 126 will thus be driven from their illustrated position. As soon as cam disc 126 closes the contact 128, the motor 114 will be energized. This motor 114, which in the apparatus of FIG. l takes the place of the servo motor 8D shown therein, will actuate the gear 75 and the drive including spindle 79 and will thus move the free end 35 of getter rod 33 closer to filament 34. This movement will be by a predetermined amount, since the cam disc protrusion 130 has a predetermined length which results in energization of the motor 114 for a predetermined time. As soon as the getter rod end 35 has the desired spacing from the iilament 34, as dictated by the adjustable reference voltage VR, the error voltage VE will become zero and the relay 115 will be de-energized and will open its contact 117. However, the cam disc 126 will still complete its particular revolution, due to the above-mentioned action of cam disc 125 and Contact 135. It is, therefore, important that the rate of revolution of discs 125 and 126 be not too low and the length of cam protrusion 131i be not to high, so that the getter rod 33 move not too close to lament 34- during any one revolution of cam discs 125 and 126. With proper dimensioning of these values, the getter rod 33 will be moved by an appropriately small increment every time its length has appreciably decreased due to vaporization of getter material therefrom.

FIG. 3 shows the high voltage supply 48 illustrated in PEG. 1. According to FIG. 3, the supply 4S is energized from a pair of leads 222 and 223. Lead 222 in connected to the stationary terminal 225 of a variable transformer 226 and lead 223 is connected to the movable terminal 22S of transformer 226. Transformer 226 is energized from the power leads 22 and 23 shown in FIG. 1.

The position of the ymovable termina-l 22S is adjusted by a servo motor 230 which has a first terminal 231, a second terminal 232, and a third terminal 233. The motor is of a design so as to rotate in one direction when terminals 231 and 232 are energized, and in the opposite direction when terminals 231 and 233 are energized. The first terminal 231 is connected to the power lead 22. The second terminal 232 is connected to one stationary terminal, and the third terminal 233 to the other stationary terminal of a switch 235 which, in turn, is connected to power lead 23.

A vacuum gauge 236 controls the position of the switch 235. The vacuum gauge 236 is responsive to the pressure in the vacuum pump 10 shown in FIG. l. If this pressure is at a desired level, the gauge 236 actuates the A switch 235 to a middle position at which neither of the motor terminals 232 and 233 is energized. If this pressure is higher Ithan desired, the gauge 236 adjusts the switch 235 so that the servo motor 230 actuates the movable arm 223 of transformer 226 in such a manner that the voltage between leads 222 and 223 is increased. If the pressure in pump 10 is at too low a level, the gauge 236 adjusts the switch 235 so that the servo motor 230 actuates the movable arm 228 of transformer 226 in such a manner that the voltage between leads 222 and 223 decreases. In this manner, thesupply of getter material in the pump is adapted to the actual pumping requirements, since the voltage between leads 222 and 223 is one of the factors which inuence the power supplied to and the rate of evaporation of the getter source in FIG. 1. Care should, of course, be taken that the motor 230 does not drive the adjustable transformer arrn 22S at too fast a rate. To this effect, the motor 230 may be of a type having a built in high reduction gear, so that the rate of adjustment of transformer arm 228 is in harmony with the pumping operation of the pump. The apparatus with which the circuit of FIG. 3 is used may be the same as illustrated in FIG. l or as modified in FIG. 2.

Various other modifications will be apparent to those skilled in the art. i

I claim:

1. In a vacuum pump, a source of vaporized material,

comprising:

(a) a rod of material to be vaporized having a free end;

(b) electron releasing means including a lament, the filament located in vicinity of the free end of the rod;

(c) means for realizing a predetermined initial spacing between the free end of the rod and the filament;

(d) an electric circuit including means for positively biasing the rod with respect to the lilament for causing electron bombardment of the free end of the ocl and vaporization of material therefrom;

(e) means for producing a voltage being proportional to the current in the electric circuit;

(f) means for producing a reference voltage;

(g) means for producing an error signal by comparing said current proportional voltage and said reference voltage; and

(h) means repsonsive to said error signal for adjusting the position of the rod with respect to the lilament.

2. 1n a vacuum pump, a source of vaporized material,

comprising:

(a) a rod of material to be vaporized having a free end;

(b) electron releasing means including a lament, the

a' filament located in the vicinity of the free end of the rod;

(c) means for realizing a predetermined initial spacing between the free end of the rod and the filament; (d) an electric circuit including means for positively biasing the rod with respect to the filament for causing electron bombardment of the free end of the rod and vaporization of material therefrom;

(e) means for measuring the current in the electric circuit;

(f) means responsive to variations of said current to produce error signals;

(g) a servo motor responsive to said error signals to produce corresponding rotary motions; and

(h) means for translating said rotary motions into corresponding linear motions for adjusting the position of the rod with repsect to the filament.

3. A source of vaporized material, comprising:

(a) a rod of material to be vaporized having a free end;

(b) electron releasing means including a filament, the filament located in the vicinity of the free end of the rod;

(c) means for realizing a predetermined initial spacing between the free end of the rod and the filament; (d) an electric circuit including means for positively biasing the rod with respect to the filament for causing electron bombardment of the free end of the rod and vaporization of material therefrom;

(e) means for producing the voltage being proportional to the current in the electric circuit;

(f) means for producing a reference voltage;

(g) means for producing an error signal when said reference voltage exceeds said current responsive voltage; and

(h) means for moving the rod toward the filament by a predetermined increment of linear motion in response to said error signal.

4. A source of vaporized material, comprising:

(a) a rod of material to be vaporized having a free end;

(b) electron releasing means including a filament, the filament located in the vicinity of the free end of the rod;

(c) means for realizing a predetermined initial spacing between the free end of the rod and the filament; (d) an electric circuit including means for positively biasing the rod with respect to the filament for causing electron bombardment of the free end of the rod and vaporization of material therefrom;

(e) means for producing a voltage being proportional to the current in the electric circuit;

(f) means for producing a reference voltage;

(g) means for producing an error signal when said reference voltage exceeds said current responsive voltage;

(h) means for producing an electric current of predetermined time duration in response to said error signal; and

(i) means responsive to said electric current of predetermined time duration for moving the rod toward the filament by an increment of linear motion proportional to said time duration.

5. A vacuum pump, comprising:

(a) a pump housing;

(b) means for ionizing gas molecules in the pump housing and for collecting ionized gas molecules; (c) a rod of getter material in the pump housing having a free end;

(d) electron releasing means in the pump housing including a filament, the filament located in the vicinity of the free end of the rod;

(e) means for realizing a predetermined initial spacing between the free end of the rod and the lament;

(f) an electric circuit including means for positively biasing the rod with respect to the lament for causing electron bombardment of the free end of the rod and vaporization of getter material therefrom into the free space in the pump housing and on inner surface portions thereof;

(g) means for sensing the current in said electric circuit; and

(h) means responsive to variations in said current for adjusting the spacing between the free end Vof the rod and the filament.

6. A vacuum pump, comprising:

(a) a pump housing;

(b) means for ionizing gas molecules in the pump housing and for collecting ionized gas molecules; (c) a rod of getter material in the pump housing having a free end;

(d) electron releasing means in the pump housing including a filament, the filament located in the vicinity of the free end of the rod;

(e) means for realizing a predetermined initial spacing between the .free end of the rod and the filament; (f) an electric circuit including means for positively biasing the rod with respect to the filament and supplying electric power and the rod and filament to cause electron bombardment of the free end of the rod and vaporization of getter material therefrom into the space in the pump housing and on inner surface portions thereof;

(g) means for sensing the electric current in said electric circuit;

(h) means responsive to variations in said current for adjusting the spacing between the free end of the rod and the filament;

(i) means for sensing pressure variations in the pump housing; and

(j) means responsive to said pressure variations for varying the electric power supplied to the rod and filament.

7. A vacuum pump, comprising:

(a) a pump housing;

(b) means for ionizing gas molecules in the pump housing and for collecting ionized gas molecules;

(c) a rod of getter material in the pump housing;

(d) electron releasing means located in the pump housing including a filament, the filament located in the vicinity of said rod for bombarding the rod with electrons and causing release of getter material therefrom;

(e) first circuit means for developing an electric signal indicative of the distance between the filament and the rod;

(f) second circuit means for providing an electric signal corresponding to a desired spacing between the filament and the rod;

(g) means responsive to said signals of the first and said second circuit means for adjusting the spacing between said filament and said rod;

(h) means for supplying electric power to said filament; and

(i) means for varying said electric power in response to pressure variations in the pump housing.

References Cited hy the Examiner UNITED STATES PATENTS 2,850,225 9/1958 Herb 230-69 2,850,662, 9/1958 Gilruth et al 230-69 X 2,954,156 9/1960 Meyer 230-69 3,024,965 3/1962 Millcron 230-69 3,074,621 1/1963 Lorenz 23S-69 3,150,817 9/1964 Jepson et al 230-69 3,150,819 9/1964 Abrams et al. 230-69 DGNLEY J. STOCKING, Primary Examiner.

MARK NEWMAN, Examiner.

V./ L. FREEH, Assistant Examiner. 

1. IN A VACUUM PUMP, A SOURCE OF VAPORIZED MATERIAL, COMPRISING: (A) A ROD OF MATERIAL TO BE VAPORIZED HAVING A FREE END; (B) ELECTRON RELEASING MEANS INCLUDING A FILAMENT, THE FILAMENT LOCATED IN VICINITY OF THE FREE END OF THE ROD; (C) MEANS FOR REALIZING A PREDETERMINED INITIAL SPACING BETWEEN THE FREE END OF THE ROD AND THE FILAMENT; (D) AN ELECTRIC CIRCUIT INCLUDING MEANS FOR POSITIVELY BIASING THE ROD WITH RESPECT TO THE FILAMENT FOR CAUSING ELECTRON BOMBARDMENT OF THE FREE END OF THE ROD AND VAPORIZATION OF MATERIAL THEREFROM; (E) MEANS FOR PRODUCING A VOLTAGE BEING PROPORTIONAL TO THE CURRENT IN THE ELECTRIC CIRCUIT; (F) MEANS FOR PRODUCING A REFERENCE VOLTAGE; (G) MEANS FOR PRODUCING AN ERROR SIGNAL BY COMPARING SAID CURRENT PROPORTIONAL VOLTAGE AND SAID REFERENCE VOLTAGE; AND (H) MEANS RESPONSIVE TO SAID ERROR SIGNAL FOR ADJUSTING THE POSITION OF THE ROD WITH RESPECT TO THE FILAMENT. 